(1) Field of the Invention
The present invention relates to a non-chromate metal surface treating composition for increasing the adhesion of a metal""s surface to any one of a group of layers applied thereto, such as corrosion resistant layers, and method of applying same. More particularly, the present invention relates to a metal trioxide, is replaced with a titanate, namely sodium metatitanate or an oxide of titanium, namely, titanium dioxide, respectively.
(2) Description of the Prior Art
It is known that solutions containing hexavalent chromium can be used to treat the surface of a metal as etching agents to increase the adhesion of layers which are subsequently applied thereto, such as protective coatings. However, although hexavalent chromium-containing solutions are efficient etching agents, they are also highly toxic and adversely affect the environment and human health. For this reason, many chromate-free solutions for treating metal surfaces have been proposed.
Thus, various non-chromate metal surface treatments, such as disclosed in Tomlinson U.S. Pat. No. 5,759,244, the disclosure of which is incorporated by reference herein, have been disclosed which can increase the adhesion of a metal""s surface to a layer subsequently applied thereto. Many of these metal treatments are based on group IV-B metals such as titanium, zirconium and hafnium. For example, U.S. Pat. No. 5,868,872 to Karmaschek et al discloses a chromium-free aqueous bath solution for non-rinse treatment of aluminum and its alloys. The solution comprises zirconium and titanium, orthophosphate, fluoride and a water-soluble or homogeneously water-dispersible organic film former. When applied, the solution is dried on the surface of the aluminum without rinsing. Similarly, U.S. Pat. No. 5,897,716 to Reghi et al discloses a chemically and thermally stable chromate-free aqueous liquid treatment for metals which increases the adhesion of protective layers to the metals"" surfaces. The chromate-free aqueous liquid comprises components selected from the group consisting of H2TiF6, H2ZrF6, H2HfF6, H2SiF6, H2GeF6, H2SnF6, HBF4, and mixtures thereof.
The shortcoming of conventional non-chromate metal surface treatments, such as those described above, is that they cannot be integrated into and employed in place of chromium-containing compounds in current metal treatment solutions which otherwise would contain chromium. As such, conventional non-chromate metal surface treatments are usually so different from previously employed chromate-containing metal surface treatments that significant changes are required to be made in the metal treating process and in the production of the metal surface treatment itself. These changes can amount substantial expenditures and usually require additional approval from Department of the Navy. Thus, there is a need for xe2x80x9cdrop-in replacementsxe2x80x9d that can be employed in place of chromium-containing compounds, such as sodium dichromate, now used in conventional chromate-containing metal treatment solutions. xe2x80x9cDrop-in replacementxe2x80x9d refers to a compound that can be employed in a metal surface treatment solution in lieu of a chromium-containing compound without requiring any or substantial changes in the make-up of the metal surface treatment process or metal surface treatment solution.
It is a primary object of the invention to provide a non-chromate metal surface treatment solution for increasing the adhesion of a layer, such as a corrosion resistant layer, to a metal""s surface wherein the solution contains a titanate or titanium dioxide in place of a chromium-containing compound.
It is a further primary object of the invention to provide a xe2x80x9cdrop-in replacementxe2x80x9d for a chromium-containing compound that can be employed in a metal surface treatment solution which otherwise would include chromium.
Another object of the invention is to provide a method of increasing the adhesion of a metal""s surface to a layer, such as a protective or corrosion resistant layer, applied thereto.
The objects of the invention are accomplished by providing a highly effective, non-chromate metal surface treatment solution which includes a titanate, such as sodium metatitanate or potassium titanate, or titanium dioxide in lieu of a chromium-containing compound in a metal surface treatment solution that otherwise would include chromium. More particularly, the invention relates to a non-chromate metal surface etching solution for etching metals, specifically, aluminum, aluminum alloys, stainless steel, titanium and titanium alloys, to increase the adhesion properties of a particular metal""s surface.
The present invention is developed on the basis of findings that the adhesion of a layer or coating, such as corrosion resistant coating, to a metal""s surface can be increased by bathing a metal substrate in an aqueous solution which contains a chromium-containing compound. Specifically, for example, it is known that a solution containing distilled or deionized water, sulfuric acid, seed aluminum and sodium dichromate dihydrate creates a superb etching solution for aluminum and aluminum alloys. It is further known that a solution containing chromium trioxide and deionized water creates a superb etching solution for stainless steel and titanium. It is believed that the chromium-containing compound in each of the foregoing etching solutions provides increased adhesion to the respective metal surface by providing a contact surface chemistry and allowing for ionic bonding.
Test results show that a metal tested without being treated with an etching solution has poor durability and weak boundry layer. For example, untreated aluminum has weak boundry layer and weak oxides; untreated stainless steel has controlled surface properties; and untreated titanium has controlled surface properties. However, since personal exposure limits (PEL) for chromates is 0.1 mg/m3 (milligram per cubic meter), chromate-containing etching solutions are not practical for use. Thus, xe2x80x9cdrop-in replacementsxe2x80x9d for chromium-containing compounds are needed for etching solutions that otherwise would contain chromium.
Sodium metatitanate, potassium titanate and titanium dioxide have been found to be well-suited as xe2x80x9cdrop-in replacementsxe2x80x9d for chromium-containing compounds in conventional metal surface etching solutions which typically include, in addition to sodium dichromate, potassium dichromate or chromium trioxide, various other less toxic or non-toxic components. The PEL of the titanium compounds is 15 mg/m3, and thus, the solutions provide highly effective, non-toxic, metal alternatives to solutions which otherwise would include chromium-containing compounds.